An exhaust gas recirculation (EGR) system is used for controlling the generation of undesirable pollutant gases and particulate matter in the operation of internal combustion engines. Such systems have proven particularly useful in internal combustion engines used in motor vehicles such as passenger cars, light duty trucks, and other on-road motor equipment. EGR systems primarily recirculate the exhaust gas by-products into the intake air supply of the internal combustion engine. The exhaust gas which is reintroduced to the engine cylinder reduces the concentration of oxygen therein, which in turn lowers the maximum combustion temperature within the cylinder and slows the chemical reaction of the combustion process, decreasing the formation of nitrous oxides (NOx). Furthermore, the exhaust gases typically contain unburned hydrocarbons which are burned on reintroduction into the engine cylinder, which further reduces the emission of exhaust gas by-products which would be emitted as undesirable pollutants from the internal combustion engine.
When utilizing EGR in a turbocharged diesel engine, the exhaust gas to be recirculated is preferably removed upstream of the exhaust gas driven turbine associated with the turbocharger. In many EGR applications, the exhaust gas is diverted directly from the exhaust manifold. Likewise, the recirculated exhaust gas is preferably introduced to the intake air stream downstream of the compressor and air-to-air after cooler (ATAAC). Introducing the exhaust gas downstream of the compressor and ATAAC is preferred due to the reliability and maintainability concerns that arise if the exhaust gas passes through the compressor is and ATAAC. An example of such an EGR system is disclosed in U.S. Pat. No. 5,802,846 (Bailey) issued on Sep. 8, 1998, which is assigned to the assignee of the present invention.
With conventional EGR systems as described above, the exhaust gas may be drawn from only a subset of the combustion cylinders within the engine, and driven back into the intake manifold. For example, the exhaust gas may be drawn from only a single cylinder of a multi-cylinder engine. In the case of a six cylinder engine, such a single cylinder EGR system would provide one-sixth of the available exhaust gases to the intake manifold, thereby providing an EGR rate of approximately 17 percent. Such an EGR rate, however, is too high under certain operating conditions, such as when the engine is operating under a peak torque condition.
Also, it is known to control the operation of the EGR valve of such conventional EGR systems by using a negative pressure source, such as an intake manifold, to sense low load conditions. However, operation of the EGR valve based on pressure changes can be adversely affected by leaks in the negative pressure source, and leaks or blockages in the control line coupling the EGR valve to the negative pressure source.
The present invention is directed to overcoming one or more of the problems as set forth above.